1. Field of the Invention
The present invention relates to a multipole lens, method of fabricating it, and charged particle beam system.
2. Description of Related Art
It is known that there are charged particle beam systems (such as transmission electron microscopes, scanning electron microscopes, and other instruments) having built-in aberration correctors for correcting aberrations (such as spherical aberration and chromatic aberration) in lenses. In such a spherical aberration corrector, aberration correction is implemented by applying a magnetic or electric field to a path for a charged particle beam (such as an electron beam) by the use of a multipole lens having a plurality of polar elements which need to be positioned with high positional accuracy.
In order to improve the positional accuracy of the multipole lens, it is desired to fabricate each individual polar element in one machining operation. For example, JP-A-2007-287365 sets forth a technique of improving the positional accuracy of a field produced by multiple polar elements by machining the front ends of the polar elements and multipole lens assembly references in one operation so as to minimize the positional deviations of the front ends of the polar elements from the assembly references.
In a charged particle beam system, in order to install such a multipole lens in a narrow space, it is necessary to reduce the parts count of the multipole lens assembly or to reduce the sizes of the parts. When the parts are reduced in size, the parts need to have simple shapes to provide high accuracy.
With the technique set forth in JP-A-2007-287365, however, one multipole lens is divided into a required number of parts. For example, where the multipole lens has 12 poles, the lens is divided into 12 parts. Mechanical components for securing these parts are also needed. This leads to an increase in parts count, in which case it is difficult to miniaturize the lens. Furthermore, where a multipole lens is fabricated from separate parts, it is difficult to position the polar elements with high accuracy.